Most communications protocols are specified using a layered architecture (e.g. using the OSI reference model). Each layer uses the service provided by the layer below. However, a layer is not always aware of the maximum size of the packet payload (Service Dat Unit (SDU)) which may be supported.
Encapsulation of a SDU by adding a PCI to form a PDU
In most cases packet networks limit the size of the maximum SDU at the network layer, but the actual maximum size will depend upon the network architecture which is being used. (LANs and MANs often allow comparatively large packets, whereas WANs often employ a much smaller maximum packet size). Many layers (e.g. the IP network protocol) therefore support a segmentation (also known as fragmentation) service, which breaks large SDUs into a number of smaller SDUs. The corresponding peer protocol is responsible for reassembling the complete SDU before forwarding to the layer above.
Segmentation of a SDU by a protocol layer
The corresponding process of joining together the received segments is known as "reassembly" and is performed by the receiver at the same protocol layer (i.e. peer-to-peer).